In a vehicle such as a hybrid vehicle, an electric vehicle, or a fuel cell vehicle, a battery unit in which a plurality of batteries are accommodated in a battery case is provided. In a battery unit described in Patent Literature 1, a battery case includes: a tray on which batteries are mounted; and a cover that covers the batteries from above, and a plurality of beam members that extend in a vehicle width direction are fixed to a lower surface of the tray. The battery unit is fixed to left and right side members through fastening portions of the beam members exposed from the tray in the vehicle width direction. In addition, in a battery accommodation portion of the tray, front-rear-direction partitions and width-direction partitions are provided in a lattice shape to be integrated with the tray. As a result, battery accommodation chambers that accommodate the batteries are divided, and the rigidity of the tray is secured.